Thermionic valve amplifier circuits



Sept. 26, 1939. w. s. PERCIVAL 2,173,914

THERMIONIC VALVE AMPLIFIER CIRCUITS Filed Dec. 31, 195"! BY v PatentedSept. 26, 1939 UNITED STATES PATENT OFFICE land, assignor to Limited,Hayes, of Great Britain Application December 31, 1937,

Electric & Musical Industries Middlesex,

England, a company Serial No. 182,651

In Great Britain January 14, 1937 9 Claims.

This invention relates to thermionic valve amplifier circuits and isconcerned with amplifiers which are required to have a considerablestage gain, while nevertheless, providing a substantially uniformamplification within a wide range of frequency.

Thermionic valve amplifiers are known for this purpose in which two ormore valves are coupled together by the elements of a low pass or bandpass network, the network being terminated by an impedance equal,approximately to the characteristic impedance of the network.

In British patent specification No. 475,490, a coupling is described forconnecting two valves together in cascade comprising three half sectionsof a low pass network terminated at one end in mid-shunt and at .theother end with the usual series m-derived termination, the anode-outputcapacity of one valve and the 20 grid-input capacity of the other valveeach forming substantially the whole of the shunt capacities of thenetwork. It is a feature of this invention that the capacity at theunterminated end of the network, i. e., the normal input end, is given avalue twice the usual mid-shunt value, 1. e., it is equal in capacity toa full shunt element. The two shunt capacities formed by the valvecapacities are thus equal and the response of the amplifier is thensubstantially flat with frequency almost up to the cut-011 frequency ofthe low-pass network.

In general, the output and input capacities of the valve will not beequal. Some adjustment can be made by arranging the circuit lay-out sothat stray capacities, e. g., of a coupling condenser, are in parallelwith either the plate or grid capacity, whichever is the smaller, inorder to make the total capacities at grid and plate more nearly equalto one another. In the case in which a considerable difference ofcapacity exists, for example, in which the capacities are in the ratioof two to one, it would not appear desirable to double the smallercapacity, since it is to be expected that addition of capacity wouldreduce the performance that could be obtained below the optimum possiblefor-the actual total capacity at anode and grid.

It is, therefore, the chief object of the present invention to providean improved thermionic 50 valve circuit in which two valves havingunequal anode and grid capacities are coupled together with a view toproviding the maximum value of pass band times stage gain Without thenecessity of adding capacity to the circuit.

According to'the invention a thermionincvalve circuit is providedcomprising two valves having unequal anode and grid capacities andwherein the anode of one of the valves is connected to the grid of afollowing valve through an inductance and to a loading impedance througha further inductance, the two inductances being coupled together andconstituting with the anode and grid capacities of the valves, sectionsof a lowpass filter network.

Preferably, the loading impedance is arranged to constitute aterminating impedance of the network and is of such a magnitude that thenetwork is substantially refiectionless. The termination is preferablyof the series m-derived type.

In the specification of co-pending patent application Serial No.171,678, filed October 29, 1937, a superheterodyne receiver is describedin which two valves are coupled in cascade through the medium of coupledcoils, forming with the output and input capacities of the valves,sections of a low pass network and the present invention may, therefore,be regarded as an extension or development of the invention described inthe specification of this co-pending patent application.

For the purpose of describing the invention more in detail, referencewill now be made to the accompanying drawing in which:

Figure 1 is a diagramof a circuit in accordance with the invention.

Figure 2 is a diagram similar to Figure 1 showing an equivalent electriccircuit,

Figure 3 illustrates the equivalent network of the circuit shown inFigures 1 and 2, and

Figure 4 is a modification of the circuit shown in Figure 1.

As shown in Figure 1, the circuit comprises a screened grid valve 5, theanode of which is connected through an inductance coil 6 and condenser lto the grid of a further screened grid valve 8, a leak resistance 9being provided as shown. The anode of the valve 5 is connected throughan inductance coil I I to a load impedance shown on the left-hand sideof the dotted line ll] which is connected to the positive terminal of asource of anode current, not shown. The two coils 6 and l I are coupledtogether and constitute, with the output capacity of the valve 5, whichis indicated in dotted lines by a condenser, and the input capacity ofthe Valve 8, also indicated in dotted lines, bya condenser, sections ofa lowpass network. The reference numeral l2 indicates the inputterminals for the valve 5, whilst the output terminals from the valve 8are indicated by the reference numeral l3. The loading impedancecomprises, in the example shown, the

i in the following explanation.

stage.

resistance l1 and a half section of the series m-derived type, formed byinductance l4, condenser l5 and inductance IS. The resistance I1 isgiven an impedance substantially equal to the nominal impedance of thenetwork so that the latter is substantially reflectionless. The hightension supply terminals indicated by the reference numeral i8 areshunted by a condenser l9.

Figure 2 of the drawing illustrates an equivalent circuit to Figure 1,from which it will be seen that the effect of coupling the coils 6 and His, in effect, to provide a further inductance indicated at 26 betweenthe junction of the coils 6 and II and the anode of the valve 5, themagnitude of the inductance 20 corresponding to the mutual inductancebetween the coils 6 and I I.

The effect of the coupling will be more fully described in connectionwith the diagram shown in Figure 3. In this figure, the condenser at theright-hand end of the network corresponds to the input capacity of thevalve 8, whilst the condenser at the middle of theh network correspondsto the output capacity of the valve 5. In this figure the inductances,condensers and resistance are indicated by symbols which will beemployed The valve 5 is a high impedance valve of the screen grid type,and thus when an alternating voltage of fixed amplitude and varyingfrequency is applied to its grid, a constant amplitude alternatingcurrent will flow in its anode circuit. The voltage applied to the gridof the following valve will be that appearing across the condenser at SFig. 3 and the variations of this voltage with frequency will be thefrequency response of the amplifier By the principle of reciprocity, theconstant current may be considered as flowing in at S. The voltagefrequency response at P will then be the frequency response of theamplifier. Regarding S as the input point, the network is seen tocomprise a series m-derived termination, one m-derived section, m havingthe value m1 for this section, a prototype half-section and a furthercapacity of value (1C. The value of the inductance and capacity of theprototype half section have the values L and C so that the capacity at Sis seen to be greater than the normal midshunt value. For the case inwhich it is desired to obtain a constant input impedance the capacity atS is made double the normal mid-shunt value and consequently the inputimpedance at S is substantially constant nearly up to the cutofffrequency as pointed out in aforesaid specification No. 475,490. Theconstant current fed in at S therefore sets up a constant voltage at Sat all frequencies substantially up to cut-off.

Now, with the usual capacity value C only at S instead of 2C, theimpedance Z5 of the filter looking in at S is the mid-shunt impedance,and, in accordance with well known theory, is resistive, and will begiven by where am being 21r times the cut-off frequency and given bywLC=1 and w is the angular frequency under consideration and R. thenominal impedance of Hence BLQ Now by the use of the capacity of value20 at S and the screen grid type of valve, Vs is constant withfrequency. Hence, substituting the values of the impedance ZQ and Zs Butfrom the theory of the m-derived filter Vp 1 o 1) "(12) Thus for m1 1the response will rise with frequency. For m1 1, which may be obtainedby mutual inductance between the coils as in Figure 1, the response willfall with frequency. The capacity at P is fixed by the valve capacityand equals 2m1C where C is the shunt capacity of the elementaryhalf-section. 0 should be as small as possible in order that the cut-offfrequency may be as large as possible for a given amplification. Henceit is necessary to have m1 1. Considering the case in which m1=\/2. Asstated this will result in, a falling characteristic which may belargely corrected by reducing (relatively) the capacity at S belowthevalue 2C. The impedance at S, and therefore the voltage, will notrise with frequency and it can be arranged that the response at P issubstantially fiat with frequency up to about 0.7 cut-off frequencywhile the pass-bandis increased. Let the capacity here be (1+a)C that isto say, the increase of capacity over the normal mid-shuntvalue is aCwhere a is between 0 and 1. The impedance at S will now be The voltageVs at S will, therefore, be proportional to this. Hence Hence usingEquation (2) above The amplitude does not vary more than 5% up.

to zv=0.7 that is to say 0.7 of the cut-off frequency.

Bydarnping the coil 6 Fig, 1, that is to. say the.

coil between the anode and grid a flatter characteristic is possible.,Th damping provided by the actual coil resistance may be sufiicient.

The ratio of capacitiesflp) .-is v v l l-a v v R In practice, the ratiois' given and it is necessary to choose m1 and hence it to satisfy this.ratio.-

In order to compare the performance of the present amplifier stage withthat described in aforesaid specification No, 475,490 in which thecapacity ratio is unity or is made unity by adding capacity tosupplement the lower valve capacity, the theoretical pass-bands may becompared for the same value of anode valve capacity, e. g., 10 b sincethis is fixed in practice. For the simple network, the pass-band andstage gain is pro-- portional to Rwo where w0=21r times the cut-offfrequency and from filter theory (3 where Co is the shunt capacity ofthe elementary half section (E F) section capacity.

mentary capacity Hence the theoretical performance of the presentamplifier is better by the factor m1.

It is assumed in the above analysis that the anode capacity is greaterthan the grid capacity. If the reverse is the case the position of thevalves may be reversed by the principle of reciprocity. The practicalcircuit for this case is shown in Figure 4 of the drawing in which theelements which correspond to the equivalent elements shown in Figure 1,bear the same reference numerals, but in this figure, instead ofemploying a series m-derived termination as shown in Fig. 1 thetermination consists of a simple resistance indicated by the referencenumeral 2|.

With the circuit of Fig. 4 which is terminated with a pure resistancethere is a tendency for the response curve to show a droop in the regioncorresponding to frequencies about half the cuton frequency. There isalso a tendency for the curve to rise in the region corresponding tofrequencies about two thirds the cut-ofi frequency. These objectionablefeatures can be corrected simultaneously by giving the mutual inductanceM between the inductance B and l l a value somewhat higher than thatcalculated in the manner indicated above.

It has also been found that in the arrangement of Fig. 4, the values ofthe self-inductance 6 and II deviate only slightly from values half anda quarter respectively of an inductance L=R C1, in which R is the loadresistance 2| as indicated above, and C1 is the total shunt capacity,that is the sum of the plate capacity of valve 5 and the grid capacityof valve 8.

It has thus been found convenient to give inductances 6 and I l valuesof L/ 2 and L/4 respectively and to alter the value of M slightly tocompensate.

If this is done, then in the case where the highest Working frequency fis selected to give a deviation of not more than 0.2 decibel in theresponse curve and in which the time delay sufficiently v nearlyconstant with frequency that at least ten stages of amplification can beused for television purposes for frequencies up to the workingfrequency, all the design data can'be calculated to a closeapproximation from the relation 1 where p is the ratio between the platecapacity of valve li and the grid capacity of valve 8, the value of theresistance 2| being given by I claim:

1. A thermionic valve circuit comprising in combination two valveshaving unequal anode and grid capacities respectively and wherein theanode of one of the valves is connected to the grid of a following valvethrough an inductance and to a loading impedance through a furtherinductance, the two inductances being coupled together and constitutingwith the anode and grid capacities of the valves, sections of a low-passfilter network.

2'. A thermionic valve circuit according to claim 1 in which the loadingimpedance is arranged to constitute a terminating impedance of thenetwork.

3. A thermionic valve .circuit comprising two valves having unequalanode and grid capacities.

respectively and wherein the anode ofone of the valves is connected tothe grid of a following valve through an inductance and-to a loadingimpedance through a' further inductance, the two inductances beingcoupled together and constituting with the anode and grid capacities ofthe valves sections of a low-pass filter network, said loading impedancebeing arranged to constitute a terminating impedance of the network, andthe anode impedance of one valve being greater than the grid capacity ofthe other, and the anode of said one valve being connected between thetwo said inductances, the grid capacity of said other valve constitutinga shunt reactance at the end of the network remote from the terminatingload.

4. A thermionic valve circuit comprising two valves having unequal anodeand grid capacities respectively and wherein the anode of one of thevalves is connected to the grid of a following valve through aninductance and to a loading impedance through a further inductance, thetwo inductances being coupled together and constituting with the anodeand grid capacities of the valves sections of a low-pass filter network,said loading impedance being arranged to constitute a terminatingimpedance of the network, the anode capacity of the one valve being lessthan the grid capacity of the other valve, the anode capacity of saidone valve constituting a shunt reactance at the end of the networkremote from the terminating load.

5. A thermionic valve circuit according to claim 4 in which the firstmentioned inductance has a self inductance value of substantially andthe second mentioned inductance a value where R is the value of theresistance in the said terminating impedance of the network and CI isthe sum of the respective anode and grid capacities, the mutualinductance between the two inductances being given a value Msubstantially determined by the expression mmw-1 o.o6 -'1m where p isthe ratio between the said grid and anode capacity, this ratio being notgreater than two.

6. A thermionic valve circuit according to claim 4 in which the firstmentioned inductance has a self inductance value of substantially andthe second mentioned inductance a value of substantially L 4. where L isgiven by L=R Ci where R is the value of the resistance inthe saidterminating impedance of the network and is substantially equal. to

and Ci is the sum of therespeotiveanode and grid capacities, the mutualinductance between the two inductances being given a value Msubstantially determined by the expression where p is theratiobetweenthe said grid and anode capacity, this ratio being nogreater than two.

7. A thermionic valve circuit comprising two valves having unequal anodeand grid capacities respectively and wherein the anode of one of thevalves is connected to the grid of a following valve through aninductance and to a loading impedance through a further inductance, thetwo inductances being coupled together and constituting with the anodeand grid capacities of the valves sections of a low-pass filter network,said loading impedance being arranged to constitute a substantiallyreflectionless terminating impedance of the network.

8. A thermionic valve circuit according to claim '7 in which the anodecapacity of one valve is greater than the grid capacity of the other andwherein the anode of the said one valve is connected between the twosaid inductances, the grid capacity of the said other valve constitutinga shunt reactance at the end of the network remote from the terminatingload.

9. A thermionic valve circuit according to claim '7 in which the anodecapacity of the one valve is less than the grid capacity of the othervalve, the anode of the said one valve being connected to the grid ofthe other valve and to the loading impedance over one of saidinductances, the other of said inductances being connected between saidone inductance and the load impedance, the anode capacity of said onevalve constituting a shunt reactance at the end of the network remotefrom the terminating load.

WILLIAM SPENCER PERCIVAL.

